I know on X10 there is an obvious advantage but this new sensor with EXR appears to act differently with noise control and DR (which is still good)

X10 has EXR so there are several advantages to shooting at M size.

X20 has X-trans which is best shot full and then reduced to suit.

DR (or highlight protection/recovery) is not related to image size on the X-Trans setup. It works the same in any size, and uses ISO amplification techniques, rather than senor pixel pairing as with EXR.